This application claims the priority of German patent documents 199 44 184.7, filed Sep. 15, 1999 and 199 47 923.2, filed Oct. 6, 1999, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to a device for evaporating liquids.
A two-stage evaporator unit in the form of a plate heat exchanger is known from DE 44 26 692 C1, in which heat exchanger plates alternate with evaporator spaces and heat-transfer spaces. The required heat of evaporation is introduced into the heat-transfer spaces with the aid of a heat-transfer medium, for example a hot heat-transfer oil. Furthermore, it is known for the heat to be generated directly in the heat-transfer spaces by catalytic conversion of a fuel.
DE 197 20 294 C1 discloses a reformer reactor with an evaporator. The reactor comprises an evaporation body which adjoins the reaction zone with surface-to-surface contact and has a porous, thermally conductive structure for providing the gas mixture which is to be reformed by mixing and evaporating the gas mixture components which are fed to it.
The object of the present invention is to provide an evaporator which is improved in terms of mass, volume, dynamics and thermal stresses.
This object is achieved by a device according to the present invention.
Designing an evaporator in the form of a porous evaporation body over which gas flows and which is directly catalytically heated has considerable advantages with regard to mass, volume, and cost. For example, it is possible to dispense altogether with the need to form additional spaces for providing the required evaporation energy. The design as a large-area layer over which gas flows allows the evaporator to be integrated in known plate-type reactors. The porous body forms a highly wettable surface which ensures that heat is introduced successfully into the liquid. Due to the porous structure, the mechanical stresses which occur during evaporation are lower than, for example, with a planar, solid metal sheet.
The vertical arrangement of the surfaces and the introduction of the liquid to be evaporated in an upper region of the evaporation body has the advantage that the force of gravity can be utilized to disperse the liquid to be evaporated inside the evaporation body. Splitting the evaporation body into an upper evaporation layer and a lower heating layer has the advantage that the pores of the catalyst material cannot fill up with liquid, which would impair operation of the device.
To produce an evaporation body, the catalyst material is advantageously pressed into a support structure. Dendritic copper powders are particularly suitable for the support structure, which powders can easily be compressed or sintered to form a mesh even if the copper powder forms a relatively low proportion of the total mass of the layer, have a large surface area and are themselves catalytically active. Therefore, the use of dendritic copper powder results in a stabilizing, fixing and heat-distributing mesh in the micrometre range.